Lecture Notes: Bill Engstrom Instructor Igneous Rocks GLG 101: Physical Geology In our overview of the Earth, we found out that Earth s internal heat combined with other mechanisms causes rocks to melt in the asthenosphere and lithosphere. The present is the key to the past = Principle of Uniformitarianism. And..We have igneous and volcanic activity going on today that we can observe to help us understand the past. NOTE: Before you start reading these notes, skim through them and locate the charts. You can use these as you read to help you remember the information and figure out how it all fits together. We will look at Igneous Rock is. A crystalline or glassy aggregate of one or more minerals formed by cooling from a molten liquid; or by consolidation of pyroclastic material (e. g. ash or other igneous fragments). Distribution of Volcanoes When we look at the distribution of volcanoes worldwide we see that there is a large concentration associated with plate boundaries (e.g. the ring of fire around the Pacific) What is Magma? Earth s internal heat combined with other mechanisms causes rocks in asthenosphere and lithosphere to melt. Magma: a molten or liquid rock or liquid crystal mix underground, often containing dissolved gases such as water, carbon dioxide, sulfur dioxide, and chlorine. Not all magma is alike there are different kinds this will be useful in interpreting igneous rocks and geologic hazards So.What is lava (and Pyroclastics)? Lava: a magma that has made it to the surface Pyroclastics: broken bits of pre existing rocks, ash, blasted out lava Igneous Rocks are classified by Texture and Composition BOTH are used. Figure 4.3 (located belowin these notes) shows a general igneous rock classification which includes basic igneous rock types. Texture refers to the size and arrangement of crystals, glass and pyroclasic material in a rock. Texture is NOT how the rock feels.
Intrusive/Plutonic textures Indicate slow cooling beneath surface Most of the crystals of intrusive rocks are visible to the naked eye Crystalline (Phaneritic (pr. Fan eritik) coarse grained) visible interlocking crystals time for crystals to grow. These are crystals that are similar in size. Porphyritic (pr. Pore feritik) Larger crystals (phenocrysts [pr. Feen o krysts) in a matrix of smaller crystals. This texture is typically formed at first by slow cooling followed by rapid cooling as the magma solidifies. Phenocrysts = the larger crystals in a porphyritic texture Groundmass = matrix = the main mass of a porphyritic texture called phaneritic in intrusives Pegmatitic Very large ( > 2 cm) crystals formed in later stages of cooling with lots of vapor. The large crystals are not simply formed by slow cooling, but are formed in a fluid rich (water and other volatiles) environment in the later stages of magma crystallization. Extrusive/volcanic textures Fast cooling at or near the surface More interlocking smaller crystals No time for most crystals to get very large Most crystals invisible to the naked eye Microcrystalline (Aphanitic [pr. Aff anitik) = fine grained) Porphyritic large crystals in a groundmass (matrix) of invisible (microcrystalline) crystals Vesicular full of vesicles (holes) where gas has escaped Frothy Texture Extrusive Glassy No time for any crystals to form Pyroclastic Glasses weld rock fragments (clasts), ash, etc. a clastic rock Composition The other ½ of our classification scheme helps us determine where the rocks were formed related to plate tectonics (tectonic environments). We will talk about rock families. Rocks that appear different because of texture may have similar compositions. Figure 4.1(at the end of these notes) shows the tectonic environments and compositions. You can follow along with that chart
while you look at the rock families. On the left side of the chart you can see how the compositions change based on Bowens Reaction Series. On the right side you can see the types of tectonic settings they occur in and the types of volcanoes and lava flows that occur. In the middle is the rock type. Some students feel that it helps to turn the chart sideways, with the left side on top and the right side on the bottom to better visualize things. Remember that Classification is based BOTH on texture AND composition AND THAT. Composition is actually what will help us figure out tectonic environments Igneous Rock Compositions Ultramafic Rock Family. Peridotite Komatiite (Ultramafic) Peridotite an ultramafic intrusive rock Komatiite an ultramafic extrusive rock Key minerals dominant olivine, minor pyroxene, minor Ca rich plagioclase Color Green to deep green Density High (Around 3.5 g/cc) Silica content (amount of Silica relative to iron and magnesium): low ~45% Temperature of melt: Extremely hot (probably > 1500 C). Viscosity of Melt: Probably very low but we don't see natural melts of this composition above the mantle anymore. Gas content of melt: Probably very low in most cases. Lava Flows & Volcanoes Youngest komatiite is older than 2 billion years. Suggests that Earth has cooled down. What is important about ultramafic rocks? Ultramafic intrusive rocks make up the upper mantle portion of the lithosphere and asthenosphere. These rocks probably formed very early Earth s history when the much of the planet was probably a giant molten mass. They are important because partial melting of mantle peridotite ultimately provides the material for all other rocks in the lithosphere. Igneous Rock Compositions Mafic Rock Family Gabbro Basalt (Mafic)
Gabbro a mafic intrusive rock Basalt a mafic extrusive rock Key minerals a few olivines, dominant pyroxene, dominant Ca rich plagioclase feldspars Color Dark in plutonic rocks an more dark crystals than light crystals Density Moderately high (Around 3.2 g/cc) Silica content (amount of Silica relative to iron and magnesium): low ~50% Temperature of melt: Very hot (~1200 1500 C). Viscosity of Melt: Low viscosity (fairly fluid relative to other magmas). Gas content of melt: low due to low viscosity of melt gas escapes easily. Lava Flows & Pyroclastics Thin widespread flows basalt (low viscosity promotes thin widespread flows) Pahoehoe (pr. Pa hoy hoy) = ropey This basalt solidifies when the most gas is still dissolved in the lava. Aa (pr. Ah ahh) = painful to walk on (when you do you might say Ah ahh ) This basalt solidifies after the loss of most of the gas from the lava. Pillow lavas (indicate underwater eruption) This basalt forms like overlapping pillows because it is solidifying under water. Pyroclastics cinders (scoria) & bombs common Volcanoes Fissure eruptions Columbia Plateau, Snake River Plain, Sunset Point AZ Shield volcanoes (Hawaii/Galapagos) Cinder cones Sunset Crater Igneous Rock Compositions Intermediate Family Diorite Andesite (Intermediate) Diorite an intermediate intrusive rock Andesite an intermediate extrusive rock Key minerals hornblende, intermediate plagioclase feldspars
Color Intermediate in plutonic rocks an equal amount of dark & light (e.g. looks like a grey/salt and pepper colored rock) Density Intermediate (Around 3.0 g/cc) Silica content (amount of Silica relative to iron and magnesium): moderate ~60% Temperature of melt: Moderately hot (900 1200 C). Viscosity of Melt: Moderately viscous. Gas content of melt: moderate percentage due to moderate viscosity of melt gas escapes with difficulty. Lava Flows & Pyroclastics Massive or blocky moderate viscosity allows flow but not very far Pyroclastics: ash and some cinder/scoria deposits common. Volcanoes Stratovolcanoes / Composite volcanoes Mt. Rainer, Cascade Volcanoes. Ash and cinders are mixed with lava flows in separate strata/layers. Stratovolcanoes are more explosive types of volcanoes due to the gas content in the magma. Many famous eruptions are the result, including Pompeii, Italy and Mt. St. Helens, Washington. Special Note: Intermediate and Felsic magmas often occur in the same tectonic areas and may even grade into each other. Consequently, they may thus occupy the same volcanoes at different times. Igneous Rock Compositions Felsic Family Granite Rhyolite (Felsic) Granite a felsic instrusive rock Rhyolite a felsic extrusive rock Key minerals Biotite, muscovite, sodium rich plagioclase, K spars, Quartz Color Light or pinkish in plutonic rocks more light (or pink) colors than dark Density Light (Around 2.7 g/cc) Silica content (amount of Silica relative to iron and magnesium): low ~75% Lava Flows & Pyroclastics
Domes Rhyolites (high viscosity prevents thin widespread flows) & Obsidian Pyroclastics: Ash Flows Rhyolite tuffs and volcanic breccias (because of extremely high gas pressures); Ash Falls (pumice) Volcanoes Stratovolcanoes /Composite volcanoes Mt. Rainer, Cascade Volanoes Collapse structures/large calderas Superstition Mtns, Crater Lake, Thera Igneous Rock Compositions REVIEW Intrusive (Phaneritic) Peridotite, Gabbro, Diorite, Granite Extrusive (Aphanitic) Komatiite, Basalt, Andesite, Rhyolite Rocks Classified by Texture Alone Frothy (may float on water) Glassy Dark colored mafic and intermediate family = Scoria Light colored felsic family = Pumice Frothy rocks are also sometimes classified by their size: Cinders, Ash, and Volcanic Bombs Purely glassy rocks are usually of the felsic family in spite of their apparent darkness. Darkness comes from impurities in the silica glass NOT from dark mafic minerals. Rock name: Obsidian
Here are some questions we need to answer. When magma is formed. What does it do? Where does it cool and crystallize to form rock? What kind of rock bodies do you get? Molten magmas are less dense and more mobile than surrounding rocks. Like hot air balloons (Diapirs), they rise out of the mantle, into the lithosphere (and eventually to the crust) Magma cools there (intrusive) or erupts on the surface to form extrusive rocks. Intrusive rocks How they cool/form Blobular Plutons or massive bodies Batholiths (tend to be dominantly diorites and granites rocks). These are large emplacements of igneous intrusive (plutonic) rock that forms from cooled magma deep in the crust. They are almost always made mostly of felsic or intermediate rock types. Batholiths are larger than stocks. The surface expressions (eroded surfaces) are usually larger than 60 square miles (100 square kilometers). Stocks are defined as smaller than that. Stocks (tend to be dominantly diorites and granites rocks). Stocks are smaller Batholiths. Laccoliths (tend to be dominantly diorites and granites rocks). These are sheet intrusions that have been injected between two layers of sedimentary rock where the pressure of the magma was high enough that the overlying strata were forced upward, giving the laccolith a dome or mushroom like form with a generally planar base. Tabular Plutons or sheet like bodies Dikes discordant tabular plutons (any type granites, diorites, and gabbros). These cut across the layers of strata. Sills concordant tabular plutons (tend to be dominantly gabbros). These are concordant with the layers of strata they are found within and are often horizontal, unless the layers have been tilted. One way to remember these are that they are flat like a window sill. Tubular Plutons cylindrical or pipe like bodies Volcanic necks discordant tubular plutons (any type granites, diorites, and gabbros)
More Principles on Relative Dating. These are important as they are used to help us determine how old the rocks are. It s a good idea to cover these now since they directly relate to igneous rocks. We will use these again later in the year. Principle of Crosscutting Relationships For the purposes of igneous rocks, this principle states that the igneous intrusion is always younger than the rock that it cuts across. Principle of Inclusions For the purposes of igneous rocks, the inclusions or xenoliths (rock fragments included in the solidified magma rock body) are older than the rock body. As magma travels it picks up pieces of rock it travels through and these rocks are found inside the magma. This principle also applies to any rock fragments included in other rock bodies, such as pebbles included in a sedimentary rock. The pebbles are older than the rock that includes them. Principle of Baked Contacts and Chilled Margins When an igneous intrusion bakes the surrounding rock, the rock that has been baked is older than the intrusion. And, The igneous rock which is closest to the margin or contact with the surrounding rock, is characterized by a zone of finer grained crystals because it was cooled more quickly (a chilled margin). Why do we find igneous rocks only in certain environments / geologic settings? What's wrong with the basic premise of the Hollywood blockbuster "Volcano about a volcano in Los Angeles? Studies of volcanoes indicate that most magmas probably originate at or near the top of the asthenosphere. Although the temperature of the asthenosphere is at or above the melting points of most silicate minerals (at the Earth's surface), it (the asthenosphere) is not a molten liquid everywhere. So..Why does it melt only at the certain tectonic settings? Because magmas are formed by. (KNOW THIS) Extra heat hot spots Introduction of water convergent boundaries Pressure reduction divergent boundaries Extra Heat Extra heat (hot spots) melt generated simply by adding extra heat.
Main mechanism at hot spots on Earth Example: Hawaii Temperature vs. Depth in the Earth Tectonic Environment of Hot Spot Volcanoes Intraplate volcanoes not on plate boundaries (Hawaii) Dominantly shield volcanoes, fissure flows (plateau or flood lavas), pahoehoe & aa flows, cinder cones and pillow lavas Magmas of basalt composition Pressure Reduction Melting at Ridges Pressure reduction melt generated by reduction of pressure on rocks already above or near their melting temperature. Main mechanism at divergent plate boundaries [e.g. as in a pressure cooker] Divergent plate boundaries (mid oceanic ridges) and continental rifts places where the continents are breaking up or are under tension (East African rift zone for example) Dominantly shield volcanoes, fissure flows (plateau or flood lavas), pahoehoe & aa flows, cinder cones Magmas of basalt composition Effect of Water on Melting Temperatures Introduction of Water Introduction of water melt generated by reducing melting temperature of rocks by introducing water. Main mechanism at convergent plate boundaries [e.g. salted roads analogy salt lowers the melting temperature of ice. The introduction of fluids water in a magma, as with salt on ice, acts to lower the melting temperature] Melting by addition of water from subducted slab at convergent plate boundaries (long volcanic chains on upper plates) Dominantly composite cones, ash flow tuffs, domes, and massive flows Magmas of andesite (mostly) and rhyolite (minor) composition.
Since the upper mantle is a Peridotite (Ultramafic) in composition, how can you melt a peridotite and get magmas of different compositions? We rely on the concept of partial melting. Recall Bowen s Reaction Series and that Different minerals form or melt at different temperatures. If you start melting a peridotite. The first liquid that forms = granitic composition (~25% melting). Further melting (~50%) produces dioritic comp. Further (~75%) produces basalt and. (100%) produces peridotite liquid. So.you can get different compositions from same parent rock by changing the amount of partial melting Example 1. 75% melting of basalt/gabbro produces a liquid of andesite/diorite composition Example 2. 75% melting of andesite/diorite produces a liquid of granite/rhyolite composition Partial Melting Plate Boundaries and Hot Spots Hot spots: Temperature Geothermal gradient=temperature increases with depth. Where we find hot spot plumes extra heat partially melts mantle peridotite to give mafic magma Divergent boundaries/continental rifts: Pressure reduction (aka. Decompression melting) also produces mafic magma from partial melting of peridotite in the mantle beneath areas of rift or divergent plate boundaries. Ex. Rio Grande Rift, Mid Atlantic Ridge. The result is the formation of Basalt/Gabbro dominant rock types/pillow lavas/plateau Basalts. Convergent boundaries: Presence of water produces intermediate magma, and the underplating of magmas produce heat to yield felsic magmas. Diorite/Andesite (intermediate) form as a result of partial melting of the downgoing basaltic slab and possible contributions from asthenosphere wedge Granite/Rhyolite form as a result of partial melting of lower crustal rocks; probably from heat introduced by magmas collecting from subduction zone. As melts ascend through crustal rocks they can become more silica rich.
Example: San Francisco Peaks, AZ This is an Andesitic (intermediate) Stratovolcano in Basalt (mafic) Volcanic Field. How can this happen? We use the concepts of Magmatic Differentiation & Crystal Fractionation and Bowens Reaction Series to help explain this. Sometimes in a dominantly basaltic volcanic field we find big andesite stratovolcanoes. It is unlikely that the plate tectonic configuration changed back and forth. Bowen's reaction series can help explain these. Magmatic Differentiation & Crystal Fractionation. As the magma cools, crystal form and settle, changing the composition of the melt. Many different crystals form and are continuously reacting with the melt. Everything we've said is true so long as crystals stay in contact with melt. If you remove crystals from melt you can stop reactions and change overall composition of the melt. OR if you add material you can change composition of melt. Crystal settling/assimilation Magmatic Differentiation = Crystal Fractionation Igneous Processes & Ore Deposits Economic aspects of igneous activity 8/2011 Crystal settling of chromite and magnetite during gabbroic magma cooling Pegmatites Gemstones (Aquamarine, emerald, corrundum, tourmaline); and lithium Hydrothermal fluids (Hot water) bearing minerals Vein deposits (Au, Ag, Cu, Fe, Pb) Magma #9 shaft/79 mine etc Underground mining techiques Disseminated Deposits (Au, Ag, Cu, Fe, Pb)